"Empathy is our ability to identify what someone else is thinking or feeling, and to respond to their thoughts and feelings with an appropriate emotion," writes Baron-Cohen. People who lack empathy see others as mere objects.

Empathy, like height, is a continuous variable, but for convenience, Baron-Cohen splits the continuum into six degrees – seven if you count zero empathy. Answering the empathy quotient (EQ) questionnaire, developed by Baron-Cohen and colleagues, will put you somewhere on the empathy bell curve. People with zero degrees of empathy will be at one end of the bell curve and those with six degrees of empathy at the other end.

Baron-Cohen provides vignettes of what a typical person with x-degrees of empathy would be like. We're told, for example, that a person with level two empathy (quite low) "blunders through life, saying all the wrong things (eg, 'You've put on weight!') or doing the wrong things (eg, invading another person's 'personal space')."

Being at the far ends of the bell curve (extremely high or extremely low empathy scores) is not necessarily pathological. It is possible to have zero degrees of empathy and not be a murderer, torturer or rapist, although you're unlikely to be any of these things if you are at the other end of the empathy spectrum – level six empathy.